JEE Math Practice Question

If $$u = \sqrt {{a^2}{{\cos }^2}\theta + {b^2}{{\sin }^2}\theta } + \sqrt {{a^2}{{\sin }^2}\theta + {b^2}{{\cos }^2}\theta } $$ then the difference between the maximum and minimum values of $${u^2}$$ is given by :

1. A.$${\left( {a - b} \right)^2}$$
2. B.$$2\sqrt {{a^2} + {b^2}} $$
3. C.$${\left( {a + b} \right)^2}$$
4. D.$$2\left( {{a^2} + {b^2}} \right)$$